1. Field of the Invention
The present invention relates to a recording apparatus such as a printer, a copying machine, a word processor, a personal computer, a facsimile machine and the like, and more particularly, it relates to a sheet conveying apparatus used with such a recording apparatus.
2. Related Background Art
An example of a sheet conveying apparatus used with a conventional recording apparatus will be explained with reference to FIGS. 11, 12A and 12B.
FIG. 11 is a sectional view of a conventional sheet conveying apparatus using manual sheet insertion supply, and FIGS. 12A and 12B are explanatory sectional views of a conventional sheet conveying apparatus in which a sheet is automatically supplied by a sheet supply roller. In these sheet conveying apparatuses, a sheet S is rested on a support plate 1 each time, and each sheet is conveyed. In the first conventional example shown in FIG. 11, the support plate 1 is secured to a frame of a sheet conveying apparatus in such a manner that a sheet stacking surface 1c of the support plate 1 is disposed on a line (convey plane) a tangent to a nip between a convey roller 4 and an opposed driven roller 5. When a thickness t of a sheet S to be used is relatively great, as shown in FIG. 11, a tip end of an upper surface Sa of the sheet S is engaged by an outer peripheral surface of the driven roller 5, with the result that a tip end of a lower surface Sb of the sheet S cannot frequently be contacted with the convey roller 4.
In such a case, an urging force T generated by operator's manual insertion is applied to the sheet S on the support plate 1 in a sheet conveying direction A, so that the driven roller 5 is lifted together with an arm 5a. As a result, the tip end of the sheet S is forcibly inserted into the nip between the convey roller 4 and the driven roller 5 to contact the tip end of the lower surface Sb of the sheet S with the outer peripheral surface of the convey roller 4. In this way, a conveying force of the convey roller 4 rotated in the conveying direction is applied to the lower surface Sb of the sheet S, thereby conveying the sheet in the conveying direction A.
In the second conventional example shown in FIGS. 12A and 12B, at an upstream side of a nip between a convey roller 4 and a driven roller 5, a sheet supply roller 13 is mounted on a frame of the apparatus in such a manner that the sheet supply roller is disposed above a sheet stacking surface 1c of a sheet support plate 1 and a lowermost point of an outer peripheral surface of the sheet supply roller can be aligned with a line (convey plane) a tangent to the nip. The support plate 1 can be lifted and lowered and is always biased toward the sheet supply roller 13 by a compression spring 3.
When a sheet S is rested on the support plate 1, as shown in FIG. 12A, the support plate 1 is temporarily lowered to a position where the sheet supply roller 13 does not interfere with the sheet S. And, when the sheet S starts to be supplied, as shown in FIG. 12B, the support plate 1 lifted to contact the sheet S with the sheet supply roller 13. Then, the sheet supply roller 13 is rotated in a conveying direction A to apply a conveying force to an upper surface Sa of the sheet S, with the result that the sheet S is conveyed in the conveying direction with a conveying force T to contact a lower surface Sb of the sheet S with an outer peripheral surface of the convey roller 4. Then, the sheet S is further conveyed in the conveying direction A by a conveying force of the convey roller 4 rotated in the conveying direction to enter the sheet into the nip between the convey roller 4 and the driven roller 5, with the result that the sheet S is further conveyed in the conveying direction A while being pinched between the rollers 4 and 5.
However, in the first conventional example shown in FIG. 11, the sheet S is supported on the support plate 1 secured to the frame of the apparatus and the position of the lower surface Sb of the sheet S is maintained on the convey plane a. Thus, when a thickness t of the sheet S to be used is relatively great, at a position where the tip end of the upper surface Sa of the sheet S is contacted with the outer peripheral surface of the driven roller 5, a distance between the tip end of the lower surface Sb of the sheet S and the outer peripheral surface of the convey roller 4 becomes great. In this case, in order to introduce the sheet S into the nip between the convey roller 4 and the driven roller 5, the sheet must be forcibly advanced in the conveying direction A with the relatively great urging force T in opposition to the biasing force of a biasing means (not shown) (for biasing the driven roller 5 against the convey roller 4) while lifting the driven roller 5. Thus, the relatively great effect is required to contact the lower surface Sb of the sheet S with the peripheral surface of the convey roller 4, and the operability is worsened.
Further, when the driven roller 5 is lifted by urging the sheet S in the conveying direction A, since stress from the driven roller 5 is concentrated at the tip end of the upper surface Sa of the sheet S, the tip end of the sheet S may be damaged or deteriorated.
On the other hand, in the second conventional example shown in FIGS. 12A and 12B, the position of the upper surface Sa of the sheet S is maintained on the convey plane a (peripheral surface of the sheet supply roller 13). Thus, when a thickness t of the sheet S to be used is relatively great, at a position where the tip end of the lower surface Sb of the sheet S is contacted with the outer peripheral surface of the convey roller 4, a distance (in the conveying direction A) between the tip end of the sheet S and the nip (between the convey roller 4 and the driven roller 5) becomes relatively great and the lower surface Sb of the sheet S is greatly spaced apart from the convey plane a. As a result, when the sheet S supported on the support plate 1 is entered into the nip between the convey roller 4 and the driven roller 5 and is conveyed by these rollers, since the sheet S is curved along the outer peripheral surface of the convey roller 4, an excessive force is applied to the sheet S, thereby deteriorating the quality of the sheet S.
Further, since the upper surface Sa of the sheet S is not regulated by the driven roller 5 until the tip end of the sheet S is entered into and pinched by the convey roller 4 and the driven roller 5, but is merely regulated by the sheet supply roller 13, when the sheet S is conveyed while being curved along the outer peripheral surface of the convey roller 4, the tip end of the sheet S cannot frequently enter into the nip between the convey roller 4 and the driven roller 5, thereby causing the poor sheet conveyance.
In addition, if the thickness t of the sheet S is relatively great and rigidity of the sheet is relatively high, even when the conveying force of the convey roller 4 acts on the lower surface Sb of the sheet S, slip may occur between the lower surface Sb of the sheet S and the convey roller 4 not to enter the sheet into the nip between the driven roller 5 and the convey roller 4, thereby causing the poor sheet conveyance.